Create a TaskRelation Object using Salesforce Enterprise WSDL

salesforce_create_taskrelation

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:urn="urn:enterprise.soap.sforce.com" xmlns:si="http://www.w3.org/2001/XMLSchema-instance">
   <soapenv:Header>
      <urn:SessionHeader>
         <urn:sessionId>NNNNNNN</urn:sessionId>
      </urn:SessionHeader>
   </soapenv:Header>
   <soapenv:Body>
      <urn:create>
		 <urn:sObjects si:type="TaskRelation">
            <!--Zero or more repetitions:-->            
            <urn:TaskId>00TO000000CyRjJMAV</urn:TaskId>
            <urn:Relation si:type="Contact">
            	<urn:Contact_ID__c>E5402055-1081-DB11-8B9B-00188B3B2CD6</urn:Contact_ID__c>
            </urn:Relation>
            <urn:IsWhat>False</urn:IsWhat>
         </urn:sObjects>  
         <urn:sObjects si:type="TaskRelation">
            <!--Zero or more repetitions:-->            
            <urn:TaskId>00TO000000CyRjJMAV</urn:TaskId>
            <urn:Relation si:type="Contact">
            	<urn:Contact_ID__c>42BD8294-0081-DB11-8B9B-00188B3B2CD6</urn:Contact_ID__c>
            </urn:Relation>
            <urn:IsWhat>False</urn:IsWhat>
         </urn:sObjects>   
      
</urn:create>
   </soapenv:Body>
</soapenv:Envelope>